Ispconfig Works perfectly on openvz. I use that on all my Servers.
Pureftpdworks fine on openvz, See ispconfig FAQ for detailed Installation instructions if your Hosting Provider has disabled some capabilities.
I recommend that you use openvz As its netter suited for Hosting Servers.
All guides that you find on ispconfig.org work for Virtual Servers.
If you use dovecot or courier is a Personal preference, both work fine.